© 2017 American Physical Society. The origin of the large thermopower in NaxCoO2 is complicated by correlation phenomena. To disentangle the effects from multiple interactions, we use angle-resolved photoemission to study KxRhO2, an isostructural analogy of NaxCoO2 with large thermopower and weak electron correlation. Using the experimentally measured electronic structure, we demonstrate that the thermopower in KxRhO2 can be quantitatively explained within the quasiparticle framework after including an electron-phonon mass enhancement effect. Extending the analysis to the cobaltate, we find the doubling in thermopower is well accounted for by additional band renormalization from electron correlation. As such, the large thermopower emerges from the itinerant quasiparticles dressed by hierarchical electron-phonon and electron-electron interactions.